1. Field of the Invention
The present invention relates to a mobile communications system such as the digital PHS (Personal Handyphone System), and to a portable terminal for use in such a system.
2. Description of the Prior Art
Conventionally, in the digital PHS system, a base station CS (cordless station) creates one data frame from four transmission slots T1 to T4 and four reception slots R1 to R4, as shown in FIG. 6. On the other hand, a terminal is allowed to use only one reception slot and one transmission slot out of those slots.
For example, a terminal PS1, when it receives a signal from a base station CS, uses a reception slot R1' that corresponds to the transmission slot T1 of the base station CS, and, when it transmits a signal to the base station CS, uses a transmission slot T1' that corresponds to the reception slot R1 of the base station CS. On the other hand, in another terminal PS2, its reception and transmission slots R2' and T2' correspond to the transmission and reception slots T2 and R2, respectively, of the base station CS.
FIG. 7 shows a sequence of steps followed when the communication with a terminal PS1 is transferred from one base station CS1 to another base station CS2. In step #1, the terminal PS1 is at location (a) shown in FIG. 5 and is "communicating", i.e. engaged in a communication session, with the base station CS1. However, as the terminal PS1 moves to location (c), the signal strength of the received signal drops significantly, and the communication condition degrades accordingly (step #2).
This causes the terminal PS1 to request the next base station CS2 to establish a link channel that can be used for a communication session and other purposes (step #3). In response, the base station CS2 allocates a link channel to the terminal PS1 (step #4). Thereafter, the terminal PS1 establishes synchronism with the base station CS2 in steps #6 and #7, establishes communication therewith through the allocated channel (steps #8 to #20), and eventually starts a communication session therewith (step #21). It takes as long as one second between steps #3 and #4; by contrast, it takes only five milliseconds between two consecutive steps from #6 to #20 (e.g. between steps #5 and #6).
In the above sequence of steps, exactly because steps #3 and #4 take a comparatively long time, these steps substantially cause termination of the communication session there. This means that it is necessary to start a new communication session again in step #21. Inconveniently, this causes the user of the terminal to be charged for a new communication session every time the communication with his or her terminal is transferred from one base station to another. In addition, it is also inconvenient that, in step #21, it takes an unduly long time to restart a communication session.